Nesting Overlay File Systems with ShadowWhiteout

Container technology improves storage efficiency by sharing base images through overlay file systems (overlayfs). Overlayfs enables temporary modifications while preserving original files by merging multiple directories into a single unified view. However, in container-in-container environments, the current overlayfs approach incurs inefficiency in terms of storage usage, as it does not support nesting due to file deletion operations. To delete a preserved original file, a special file is created indicating that the file is deleted in the overlayfs, but this mechanism lacks expressiveness for nesting. This paper introduces ShadowWhiteout, a new representation for file deletion in overlayfs. We identify key technical challenges and implement a ShadowWhiteout-capable overlayfs. Our system allows nesting overlayfs while maintaining reasonable runtime overhead.